Conventionally, packaging materials such as antirust papers, and synthetic resin films tempered with rust preventive agents have been used for packaging a steel coil. However, recently, a new technology has gained attention, in which a rustproof stretch film containing preservative is utilized so as to make a transparent film with self-adhesive properties (pressure sensitive adherence) and stretchability for automatic packaging of a steel coil.
Japanese unexamined publication No. 2000-326455 discloses a rustproof stretch film comprising three layers in which an intermediate or a core layer is composed of a low density polyethylene resin with a density of 0.920–0.935 g/cm3 mixed with volatile rust preventive agents, a film composed of an ethylene-vinyl acetate copolymer resin is laminated to the first outer layer (surface layer) of the intermediate layer, and a film of polyolefin resin is laminated to the second outer layer (surface layer) of the intermediate layer. In this rustproof stretch film, one surface of outer layers, which is composed of an ethylene-vinyl acetate copolymer resin, functions as an adhesive layer. Moreover, Japanese unexamined publication No. 2001-341238 discloses a rustproof stretch film comprising two-layers in which a rustproof layer is formed from materials in which an ethylene-α-olefin copolymer resin with a density of 0.890–0.920 g/cm3 is mixed with volatile rust preventive agents, and an adhesive layer is formed from ethylene-vinyl acetate copolymer resin containing liquid aliphatic hydrocarbons as adhesive agents.
When a steel coil is automatically packaged utilizing a rustproof stretch film, such a stretch film used as a packaging material has been generally taken up to a roll. The width of such a roll (the width of a film) is normally 25 cm. In an automatic packaging apparatus for a steel coil, when a roll to which a rustproof stretch film has been taken up is stored in a self-propelled cartridge that rotates along an elliptic orbit passing through a hollow part of a steel coil, the rustproof stretch film in a tape form is continuously pulled out from the take-up roll arranged in the self-propelled cartridge so as to wind around the surface of the steel coil with its adhesive face inside. The steel coil that is supposed to be packaged rotates in a fixed direction along an axis of itself as it interlocks with the rotation of the self-propelled cartridge in the elliptic orbit so that the both end surfaces, external surface, and internal surface of the steel coil can be packed with the rustproof stretch film in the tape form equally and thoroughly.
The adhesion strength of a rustproof stretch film determines not only the operability of the automatic packaging but also whether or not rusting of a steel coil can be successfully prevented. The adhesion of a rustproof stretch film is considered to include two different kinds of adhesion, i.e., a roll tack (adhesion 1) and a self-tack (adhesion 2).
A roll tack involves adhesive resistance at the time when a tape-type film is pulled out from a take-up roll arranged in a self-propelled cartridge. The strength of the roll tack represents the strength of the pull out resistance, and therefore, the greater the roll tack is, the greater the pull out resistance is. Hence, the operability upon the automatic packaging is closely connected to the roll tack of the rustproof film. For example, when the roll tack of the rustproof stretch film is excessive, a neck-in which occurs at the time of pulling the tape-type film from the take-up roll associated with the rotation of the self-propelled cartridge becomes large so that the overlapping width of films becomes small, which may break the film and cause the interruption of the packaging operation. Further, when the roll tack of the film is deficient, a change of speed of a self-propelled cartridge during a packaging operation or abrupt deceleration at the end of the packaging operation causes a film to be pulled out excessively from the take-up roll by the inertia brought by rotation of the take-up roll and to become twisted, which prevents continuous operation.
On the other hand, a self-tack of a rustproof stretch film involves the adhesion between films that have packed a steel coil or the like. The greater the self-tack is, the firmer the films taken up to a steel coil adhere to each other after packaging. Hence, a self-tack of a rustproof stretch film relates to the rust preventative capacity of the packaging material. Namely, when a rustproof stretch film with a great self-tack is used, the chance that moisture infiltrates into a gap between laminated films becomes less, which allows the preservation ability to perform better. Further, when a rustproof stretch film includes volatile rust preventive agents, not only the volatilization of the rust inhibitor to external environment becomes less but also the infiltration of the moisture becomes less, so that the preservation ability performs even better.
Conversely, when the self-tack of the film is deficient, the film that is a packaging material is liable to come off a steel coil, which will lose the satisfactory rust preventative effect of the steel coil.